Mimosa, or silk tree or silky acacia, was introduced to the Americas in the 1700s and now thrives in the zones 6 to 9. Where winters are mild, little pruning is needed for this vigorous and invasive tree. Toward the northern part of its American range, heavy pruning in early spring helps mimosa trees recover from winter die-back.
Clip off root suckers with pruning shears down to ground level to control mimosa's invasive habits. Unchecked, mimosa can become a dense thicket. Sucker shoots spring up well away from the tree, as roots spread beneath yard and garden.
Prune broken limbs as soon as damage is spotted. Limb loppers work well on any limbs which fit the jaws; mimosa wood is soft. Young trees require no special growth training. Mimosa's natural shape is tall and spreading, with an open network of branches. In areas with warm winters, even mature trees need little attention beyond removing an occasional damaged branch.
Prune dead wood from winter-damaged mimosa in early spring. Use a pruning saw or limb lopper on light damage. Chainsaws will be needed if severe winter cold kills the tree back to the ground. Even mimosa that have died back to ground level usually recover quickly. If the trees don't show life in the spring, cut out dead wood immediately. Winter-killed limbs and trunks will soon be lost in the thicket of green sprouts.
Shape winter-killed mimosa trees by removing all but the strongest shoot that emerges. Saving two or more sprouts creates an arched cluster spreading outward from the stump. Trees quickly grow to nearly their original height. Cutting back to a single main trunk provides better access for mowing.
Prune mimosa limbs overhanging roofs in late winter when the tree is dormant. With a pole saw, cut back to a strong fork in the branch or all the way back to the main trunk if necessary. Mimosa creates large amounts of leaf litter, blossom falls, and seed pods and cause a lot of gutter problems throughout the growing season if not cut back.